1. Field of the Invention
The present invention relates generally to hybrid motor vehicles and, more particularly, to a hybrid powertrain system including a power take-off driven vehicle accessory.
2. Background of the Invention
Mechanical powertrain systems commonly employ a power transmission mechanism driven by a prime mover, such as an internal combustion engine, to power a primary vehicle drive shaft. Equally common are arrangements in which a secondary drive shaft is coupled to the prime mover through some sort of transmission power take-off mechanism, thereby enabling the prime mover to independently power a vehicle accessory in addition to one or more of the vehicle wheels.
Many heavy vehicles in use today (e.g., utility trucks, over-the-road tractors and the like) include at least one power take-off (“PTO”). In utility trucks, for example, a PTO may be used to drive a hydraulic pump in an on-board vehicle hydraulic system. In some configurations, a PTO driven vehicle accessory may be powered while the vehicle is moving. In other configurations, a PTO driven accessory may be powered while the vehicle is stationary and the prime mover is operating. Still others may be driven while the vehicle is either stationary or traveling. Regardless of the type of PTO configuration used, ease of operator use and flexibility of control are important considerations when designing a vehicle's PTO driven accessory system.
Motor vehicle manufacturers are also actively working to develop alternative powertrain systems in an effort to reduce the level of pollutants exhausted into the air by conventional powertrain systems equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles. Unfortunately, these alternative powertrain systems suffer from several disadvantages and, for all practical purposes, are still under development. However, “hybrid” vehicles that include an internal combustion engine and an electric motor offer a compromise between traditional internal combustion engine powered vehicles and full electric powered vehicles. A hybrid powertrain systems' unique configuration allows the electric motor to provide the sole power needed to operate one or more vehicle functions that include, among other things, independent delivery of motive power to the vehicle wheels through the power transmission mechanism. While hybrid powertrain systems continue to improve in efficiency and seamlessness of operation, a need remains for an improved hybrid powertrain system that allows operation of a power take-off driven vehicle accessory independent of the internal combustion engine.